First generation optical sensors are currently being introduced in biotechnological production platforms. The sensors are composed of five different units, excluding the fiber optical connectors: i) light source, ii) substrate, iii) matrix, iv) indicator dye components, and v) detector.1-5 
Light sources and detectors are highly developed and is just a question of costs. The substrate has to be chosen based on the platform in which the sensing will take place, typically a glass or a polymer support is used. The key parameter regarding the substrate is that the matrix material must be able to be at least partly immobilized in or on the substrate.
The wish list for the matrix material is long: the matrix material should allow the analytes to pass through the film as unhindered as possible, it should encapsulate the sensor molecules, it should be transparent and have a low auto-fluorescence, and it has to be stable in biological media for extended periods of time. The indicator dye components may be either a single ratiometric pH responsive dye, or two dyes with similar properties. The latter is only possible if the physical stability of the matrix ensures that no dye is lost to the medium.
The benchmark in materials for optical sensors has been set in sensors, where fluorescein has been used as the indicator dye component4-8 despite the poor photostability of fluorescein.5 The critical parameters are the response time of the sensor, the leakage of the dye, the stability of the signal and the response to pH. While leakage of the highly water soluble fluorescein from the prior art optical sensors has not been completely removed,9 other more lipophilic dyes have been successfully encapsulated in sol-gel matrices.10, 11 However, even lipophilic dyes may be prone to leakage during long term use or in lipophlic/amphiphilic environments.
Preparation of organically modified silicates (ORMOSILs) using alkyl and 3-glycidoxypropyl substituted trialkoxysilanes and various polymerization conditions have been reported previously in the scientific literature.1, 12, 13 Leakage has been controlled either using apolar additives11, 14 or by attaching the dyes to bulky macromolecules.6, 7, 15, 16 It has been reported that Lewis acids can be a catalyst for polymerization of 3-glycidoxypropyltrialkoxysilanes, accelerating both the polyether and the polysiloxane formation.12, 13, 17 
WO 2009/020259 discloses in example 2 a method in which 3-glycidoxypropyltrimethoxysilane (GPTMS), methyltriethoxysilane (MTES), ethanol (6.95 mM) and 35% HCl were mixed together and stirred at room temperature for three days to induce a condensation reaction. To the sol-gel solution thus prepared, 1 mM HPTS solution, which had been dissolved in ethanol was added to give a HPTS mixture solution. The HPTS mixture solution was evenly coated onto the bottom surface of wells of a microtiter plate to prepare a fluorescent sensing membrane that can be used for detection of carbon dioxide. The sol-gel solution comprising coated HPTS was dried at room temperature for five days and further dried at 70° C. for two days for improving a mechanical strength and surface smoothness. WO 2009/020259 uses HCl as the initiator and the indicator moiety (HPTS) is non-covalently attached to a silane.
WO 2004/077035 discloses a CO2 sensor comprising a pH-indicator and a porous sol-gel matrix. The pH-indicator may be hydroxypyrene trisulfonate (HPTS) and immobilised in the sol-gel. The sol-gel may be prepared from the monomer ethyltriethoxysilane (ETEOS). In the specific method, two silanes are used (trimethylsilylpropane and triethoxysilane). However, none of the silanes suggested in the description contains an epoxy group. Furthermore, the indicator moiety is not covalently linked to a silane.
WO 12/032342 discloses a sensor comprising a sol-gel layer incorporating a phosphorescent material, such as ruthenium oxide (Ru02). The sensor may be used for measuring the O2 or the H2S concentration. Details on the monomers used in the sol-gel are not disclosed.
J. Mater. Chem. 2012, 22, 11720 shows a method in which two monomers (ETEOS and GPTMS) are used in the sol-gel. The monomers are separately reacted and methylimidazole is used to initiate the reaction of GPTMS. When the separately reacted monomers are mixed, the indicator moiety (HPTS) is added. Thus, a Lewis acid for initiating the reaction is not used and an indicator moiety (e.g. HPTS) is not covalently attached to a silane. Methods based on catalysis by methylimidazole may be inferior, as tests performed by the present inventors have shown that methylimidazole reacts and form fluorescent compounds, which are immobilized in the sol-gel.
It is the purpose of the present invention to improve the porosity of sol-gel materials for optical sensing, while at the same time maintaining a high physical stability and a low auto-fluorescence. A high porosity results in a short response time, which makes it possible to react on a change faster.